The Nankai Trough Seismogenic Zone Experiment (NanTroSEIZE) project is a multiexpedition, multistage Integrated Ocean Drilling Program (IODP) drilling program focused overall on understanding the mechanics of seismogenesis and rupture propagation along subduction plate boundary faults (Tobin and Kinoshita, 2006a). The drilling program includes a coordinated effort to characterize, sample, and instrument the plate boundary system at several locations offshore the Kii Peninsula (Fig. F1) culminating in drilling, sampling, and instrumenting the plate boundary fault system near the updip limit of inferred coseismic slip, at 5–7 km below seafloor (Tobin and Kinoshita, 2006b) (Fig. F2). The main objectives are to understand

  • The mechanisms controlling the updip aseismic–seismic transition along the megathrust fault system;

  • Processes of earthquake and tsunami generation and strain accumulation and release, including the role of recently discovered slow slip and very low frequency earthquakes (e.g., Ito and Obara, 2006);

  • The absolute mechanical strength of the plate boundary fault and its degree of interseismic locking; and

  • The potential role of a major upper plate fault system (termed the "megasplay" fault) in seismogenesis and tsunamigenesis.

The drilling program is evaluating a set of core hypotheses through a combination of riser and riserless drilling, long-term observatories, and associated geophysical, laboratory, and numerical modeling efforts. The following hypotheses are paraphrased from the original IODP proposals and outlined in Tobin and Kinoshita (2006a, 2006b):

  1. Systematic, progressive material and state changes control the onset of seismogenic behavior on subduction thrust faults.

  2. Subduction megathrusts are weak faults.

  3. Plate motion is accommodated primarily by coseismic frictional slip in a concentrated zone (i.e., the fault is locked during the interseismic period).

  4. Physical properties of the plate boundary system (including the fault system and its hanging wall and footwall) change with time during the earthquake cycle.

  5. A significant, laterally extensive upper plate fault system (the megasplay fault; Park et al., 2002) slips in discrete events that may include tsunamigenic slip during great earthquakes. It remains locked during the interseismic period and accumulates strain.

At Nankai, high-resolution seismic reflection profiles across the outer rise clearly document a large out-of-sequence thrust fault system (the megasplay fault, after Park et al., 2002) that branches from the plate boundary décollement close to the updip limit of inferred coseismic rupture in the 1944 Tonankai M 8.2 earthquake (Fig. F2). Several lines of evidence indicate that the megasplay system is active and may accommodate a significant fraction of plate boundary motion (Moore et al., 2007, 2009). However, the partitioning of strain between the lower plate interface (the décollement zone) and the megasplay system and the nature and mechanisms of fault slip as a function of depth and time on the megasplay are not understood. One of the first-order goals in characterizing the seismogenic zone along the Nankai Trough—and which bears both on understanding subduction zone megathrust behavior globally and on defining tsunami hazards—is to document the role of the megasplay fault in accommodating plate motion (both seismically and interseismically) and to characterize its mechanical and hydrologic behavior.

IODP Site C0002 was selected as the primary site to target the megasplay and top of subducting basement reflectors in the seismic volume, in order to sample all the presumed fault zones that comprise the plate boundary, make downhole measurements of their physical properties, and place long-term monitoring instruments in and around the fault zone.

Site C0002 is the centerpiece of the NanTroSEIZE project, intended to access the plate interface fault system at a location where it is believed to be capable of seismogenic locking and slip and to have slipped coseismically in the 1944 Tonankai earthquake. The primary targets include both the basal décollement and the reflector known as the "megasplay fault" (Tobin and Kinoshita, 2006b). The megasplay fault zone and the accretionary prism domain are the location of a newly identified class of earthquakes known as very low frequency earthquakes (Ito and Obara, 2006) as well as the first observation of shallow tectonic tremor (Obana and Kodaira, 2009). The megasplay fault reflector lies at an estimated depth of 5000~5200 meters below seafloor (mbsf), and the top of the subducting basement is estimated to lie at 6800~7000 mbsf (Fig. F3).

During IODP Expedition 314, Site C0002 was drilled to 1401 mbsf with in situ measurement of physical properties and borehole imaging through logging while drilling (LWD) but no coring (Expedition 314 Scientists, 2009). Several months later, portions of Site C0002 were cored over the intervals 0–204 and 475–1057 mbsf during IODP Expedition 315 (Expedition 315 Scientists, 2009). Lithostratigraphy at Site C0002 is characterized by turbiditic sediments to ~830 mbsf, underlain by older rocks of the accretionary prism and/or early slope basin sediments deposited prior to the development of the megasplay fault.

Further background, objectives, and accomplishments to date for the NanTroSEIZE project are discussed in Tobin et al. (2009), Expedition 319 Scientists (2010), and Underwood et al. (2010).